I use the Wahoo Kickr and I tried a Workout on Zwift for the first time. When I reached a 10sec interval at 500watts it took a while for the resistance to go down when I reached the 1 min interval at 145watts. It took almost 30sec before it went to the normal resistance
Do a spin down calibration on the Wahoo fitness App 1st, then close App, now start Zwift and it should be fixed.
Some trainers are more responsive from others but 30 seconds is way to long. Did this happen only once? Also, are you running Zwift only or any other apps too?